Because accountants mostly.

For large businesses, you essentially have two ways to spend money:

• OPEX: "operational expenditure" - this is money that you send on an ongoing basis, things like rent, wages, the 3rd party cleaning company, cloud services etc. The expectation is that when you use OPEX, the money disappears off the books and you don't get a tangible thing back in return. Most departments will have an OPEX budget to spend for the year.
• CAPEX: "capital expenditure" - buying physical stuff, things like buildings, stock, machinery and servers. When you buy a physical thing, it gets listed as an asset on the company accounts, usually being "worth" whatever you paid for it. The problem is that things tend to lose value over time (with the exception of property), so when you buy a thing the accountants will want to know a depreciation rate - how much value it will lose per year. For computer equipment, this is typically ~20%, being "worthless" in 5 years. Departments typically don't have a big CAPEX budget, and big purchases typically need to be approved by the company board.

This leaves companies in a slightly odd spot where from an accounting standpoint, it might look better on the books to spend $3 million/year on cloud stuff than $10 million every 5 years on servers